I tried to use flash the image under Win10 onto a Sandik Ultra 256 GB card. First aof all the card was about 238MB too small.
So I mounted the image under Ubuntu and deleted some PSX roms, then shrunk the image. So far so good.
After flashing the image, a Pi 3B constantly shows an undervoltage error and a Pi3B+ shows activity on the LEDs but
never shows more than a black screen. (Power supply delivers 3000mA).
After flashing a clean, empty retropie image onto the same card both devices boot as they should.
What am I doing wrong here? Can someone help me with this?